11

Applications Information

LED Drive Circuit Considerations For Ultra High CMR

Performance

Without a detector shield, the dominant cause of op-

tocoupler CMR failure is capacitive coupling from the

input side of the optocoupler, through the package, to

the detector IC as shown in Figure 15. The ACNV4506

improve CMR performance by using a detector IC with

an optically transparent Faraday shield, which diverts the

capacitively coupled current away from the sensitive IC

circuitry. However, this shield does not eliminate the ca-

pacitive coupling between the LED and the optocoupler

output pin and output ground as shown in Figure 16.

This capacitive coupling causes perturbations in the LED

current during common mode transients and becomes

the major source of CMR failures for a shielded optocou-

pler. The main design objective of a high CMR LED drive

circuit becomes keeping the LED in the proper state

(on or off) during common mode transients. For example,

the recommended application circuit (Figure 14), can

achieve 30 kV/

µs CMR while minimizing component com-

plexity. Note that a CMOS gate is recommended in Figure

14 to keep the LED off when the gate is in the high state.

Another cause of CMR failure for a shielded optocoupler

is direct coupling to the optocoupler output pins through

magnitude of the direct coupling including: the position

of the LED current setting resistor and the value of the

A high CMR LED drive circuit must keep the LED on during

common mode transients. This is achieved by overdriv-

ing the LED current beyond the input threshold so that it

is not pulled below the threshold during a transient. The

recommended minimum LED current of 10 mA provides

Figure 1) to achieve 30 kV/

µs CMR.

The placement of the LED current setting resistor effects

the ability of the drive circuit to keep the LED on during

transients and interacts with the direct coupling to the

optocoupler output. For example, the LED resistor in

Figure 17 is connected to the anode. Figure 18 shows the

AC equivalent circuit for Figure 17 during common mode

by an amount equal to the current that flows through

the output high (toward a CMR failure) at the same time

the LED current is being reduced. For this reason, the rec-

ommended LED drive circuit (Figure 14) places the current

setting resistor in series with the LED cathode. Figure 19

Figure 14. Recommended LED Drive Circuit

Figure 15. Optocoupler Input to Output Capacitance Model for Unshielded

Optocouplers

*100 pF TOTAL

CAPACITANCE

310Ω

+5 V

CMOS

0.1µF

+

9

2

8

3

7

4

SHIELD

5

1

6

10

20kΩ

9

2

8

3

7

4

5

1

6

10

9

2

8

3

7

4

5

1

6

10

20kΩ

9

2

8

3

7

4

SHIELD

5

1

6

10

20kΩ

Figure 16. Optocoupler Input to Output Capacitance Model for Shielded

Optocouplers

Figure 17. LED Drive Circuit with Resistor Connected to LED Anode

(Not Recommended)

*100 pF TOTAL

CAPACITANCE

310Ω

+5 V

CMOS

0.1µF

+

9

2

8

3

7

4

SHIELD

5

1

6

10

20kΩ

9

2

8

3

7

4

5

1

6

10